This invention relates generally to oil and gas well logging tools. More particularly, this invention relates to an improved method of analyzing signals produced by nuclear logging tools typically used for determination of formation density and porosity and mineralogy.
In petroleum and hydrocarbon production, it is desirable to know the porosity of the subterranean formation which contains the hydrocarbon reserves. Knowledge of porosity is essential in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of porosity is particularly useful in older oil wells where porosity information is either insufficient or nonexistent to determine the remaining in-place oil and to determine whether sufficient oil exists to justify applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas.
If the density of the formation is known, then porosity can be determined using known equations. A variety of tools exist which allow the density of the reservoir to be determined. Most of these tools are effective in determining the density (and hence porosity) of the reservoir when the wellbore in which the tool is run is an uncased reservoir and the tool is able to contact the subterranean medium itself. However, once a well has been cased, there exists a layer of steel and concrete between the interior of the wellbore where the tool is located and the formation itself. The well casing makes it difficult for signals to pass between the tool and the reservoir and visa versa. In addition, the cement can confuse the measurement of formation properties.
Common to the logging tools generally referred to as “nuclear” logging tools is a source of radiation. The emitted radiation interacts with the earth formation and the results of the interaction, which may be neutrons or gamma rays are detected (as pulses) by one or more detectors. Analysis of the numbers of pulses having certain amplitudes corresponding to various energy levels of gamma rays can provide information about the presence of certain elements or isotopes. A graphic representation of the number of pulses occurring with respect to the energy level of the pulses typically displays localized maxima, called “peaks” at several energy levels within the energy range of the scintillation detector, which typically is some portion of the range of 0.1 to 10 million electron volts (MeV), depending on the crystal type and the elements intended to be resolved. The peaks also have a range of energy levels characteristic to the isotope.
The amplitudes of the voltage pulses are typically analyzed by using a device called a spectral analyzer. The spectral analyzer comprises a pulse height quantizer for measuring the amplitude of each voltage pulse from the photomultiplier, and a storage device for counting the number of voltage pulses of each magnitude determined by the quantizer. Based on the amplitude measurement made by the quantizer, a quantization value called a channel number is assigned to each measured pulse. Each pulse leaving the quantizer increments a particular storage buffer in the storage device corresponding to the channel number determined for each pulse by the quantizer. At the end of any measurement period, the number of events counted in each buffer is used for analysis.
These spectra have to be calibrated so that a specific channel represents a certain energy range. This has traditionally been done by using various peak finding routines to locate spectral peaks and then using a least squares fit to determine the gain and offset needed to map these peaks into the desired energies. One difficulty with this technique is that the peak finding routines can confuse statistical variation with spectral peaks. Channel-to-channel filtering of the data is often required to prevent this problem. Unfortunately there can be a loss of resolution in the spectra when this is done.
The present invention addresses the problems inherent in the use of peak-finding techniques for channel calibration.